Halation has been a persistent problem with photographic films formed by depositing one or more layers of a photosensitive silver halide emulsion onto a support such as a plastic film. The emulsion diffusely transmits light. Such light reaches the support and is reflected back into the emulsion. The silver halide emulsion is thereby reexposed at a location different from the original one. The result is a halo surrounding an image of a bright object on the film.
Three methods have been proposed for preventing such halation. One such method calls for dyeing the support, another for providing a dyed or pigmented layer behind the clear support as an antihalation backing. A third method involves providing a silver antihalation layer between the support and the photosensitive layer(s). Gray silver has been used for this purpose; see James, The Theory of the Photographic Process, 4th Ed., p. 579. The process of making a grey antihalation layer involves precipitation of silver chloride and the addition of a fogging developer to produce filamentary silver similar to the morphology customarily encountered when conventional silver halide films are processed in a B/W developer.
Halation reduces the sharpness of the resulting image. Efforts to minimize the effects of halation have included incorporation of silver or silver halide in the support to lower reflection density. See U.K. Patent No. 1,126,797. Schadt U.S. Pat. No. 4,460,679, issued Jul. 17, 1984, approaches the problem by adding a nonphotosensitive layer containing a chemically bleachable, high strength tinctorial colorant, such as blue colloidal silver, over the support which permits sensitometric control over low coating weight silver halide elements. Ohbayashi U.S. Pat. No. 4,563,406, issued Jan. 7, 1986, places a yellow, blue or gray colorant layer, a white pigment layer and a silver halide emulsion layer over the support to achieve the combined effect of increased sharpness without loss of sensitivity.
Pechmann U.S. Pat. No. 2,921,914, issued Jan. 19, 1966, uses a blue colloidal silver dispersion to absorb longer wavelengths of light for halation control. Preparation of this dispersion involves spontaneous nucleation to derive centers for silver formation, and reduction of the silver salt by tannic acid in the presence of a water soluble strontium salt. This technique has hue control problems related to large nuclei, turbidity and difficulties with bleaching. Severe gel slugging (forming hardened globs), a major concern in antihalation silver preparation, is aggravated by the use of tannic acid. Posse U.S. Pat. No. 3,333,960 issued Aug. 1, 1967, discloses mixing of the Pechmann blue silver with yellow Carey-Lea silver to obtain a neutral coloration which controls halation. Neither approach is completely effective, particularly at low coverage levels. Blue colloidal silver materials made by development of fine silver nuclei have been proposed for use in halide-ion sensitive imaging systems and thermal imaging systems. See Defensive Publication T 900,010 and copending U.S. patent application Ser. No. 07/344,950, filed Apr. 28, 1989, the entire content of which is incorporated by reference herein. The present invention advantageously applies such materials to form antihalation layers displaying improved performance.